1. Field of the Invention
This invention relates broadly in the field of electronic musical tone generators and in particular is concerned with the provision for a digital noise generator in a musical instrument.
2. Description of the Prior Art
While the majority of musical tones are characterized by a well-defined pitch, there exists a large group of musical instruments which have no clearly defined pitch characteristic. Such instruments generally are members of the percussion family. These include drums, cymbals, maracas, wood blocks, and tamborines. It is well known in the electronic music art that these "unpitched" musical sounds can be imitated using a random noise generator as the primary signal source. Examples of the application of random noise generators in an electronic musical instrument are contained in U.S. Pat. No. 3,247,307 entitled "Electronic Musical Instrument."
The most common characteristic of most noise generators is that they produce a noise signal of the type given the generic name of "white noise." White noise can be defined as a signal which is uniformly and randomly distributed in amplitude and has a power spectrum which is constant per unit bandwidth over the entire frequency region. No musical instrument, or in fact any real physical device, has a signal characteristic that approaches that of a white noise signal type. An unpitched musical instrument's signal output has a power spectrum that tends to fall off at high frequencies in a manner similar to the response shape of a low-pass filter.
Noise signal sources having a power spectrum that is not white are frequently described by the generic name of "pink noise." A more limited use of the term pink noise has been applied to the special case of a noise signal whose spectrum contains constant power per percentage bandwidth for all frequencies. The broader generic definition of pink noise will be used in the following descriptions.
Pink noise generators have been used in conjunction with the tone generators of the analog variety and it is evident that pink noise generators are also desirable adjuncts to musical tone generators of the digital variety. A method for generating an analog noise signal is described in the technical article: D. B. Keele, Jr., "The Design and Use of a Simple Pseudo Random Pink-Noise Generator," J. Audio Engineering Society, Vol. 21 (January/February 1973), pp. 33-41. The system described in the article starts with a conventional shift register variety of a binary white noise generator. The output random sequence of "0" and "1" signal states from the shift register are transformed by an analog filter to produce the desired pink noise signal.
It is obvious to those skilled in the art that the binary white noise generator output signals from a shift register noise generator can also be processed by a digital filter to produce a source of digital pink noise. One disadvantage to this conventional and straightforward approach is that the use of a digital filter is not a simple and low cost system implementation. The high cost of a digital filter is a result of the need to use one or more digital data multipliers.
A digital noise generator is described in the copending patent application Ser. No. 95,896 filed on Nov. 19, 1979 and entitled, "A Noise Generator For A Polyphonic Tone Synthesizer." This application issued as U.S. Pat. No. 4,270,430. This application and the present invention have a common assignee. The noise generator disclosed in the copending application is specifically directed to producing noise-like signals having adjustable spectral characteristics to be utilized in a polyphonic tone synthesizer of the type described in U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer." The system operates by providing a set of randomly generated harmonic coefficients which are employed during a computation cycle during which a master data set of equally spaced waveshape points are computed. The master data set is transferred to a note register in a manner such that the generation of the musical instrument's tone is not interrupted. The data residing in the note register is read out sequentially and periodically at a rate determined by an adjustable frequency clock. The output data is converted to positive values and transformed to an analog signal by means of a digital-to-analog converter. Provisions are incorporated for varying the spectral content of the resulting noise-like signal by an adjustable formant subsystem.
It is a feature of the present invention that pink noise with a wide variety of spectral shapes can be generated directly without the use of either an analog or digital filter. The underlying mathematical theory of the noise generator of the present invention is essentially contained in the two articles: C. T. Mullis and K. Steiglitz, "Circulant Markov Chains as Digital Signal Sources," IEEE Trans. Audio Electroacoust., Vol. 20, October 1972, pp. 246-248; L. J. Siegel, K. Steiglitz, and M. Zuckerman, "The Design of Markov Chains for Waveform Generation," IEEE Trans. Audio Electroacoust., Vol. 24, December 1976, pp. 558-562.
The above referenced articles provide the theoretical background for the use of circulant markov chains as a noise generation source but no practical means is described for implementing a pink noise generator in a viable system form. The present invention provides a novel means for producing digital pink noise signals with adjustable spectral responses without using conventional digital filters. It is a feature of this invention that the noise spectral characteristics can be made time variant in a simple and controlled fashion.